1. Field of the Invention
The present invention relates,, to a thermal printer having a paper feeding mechanism which employs a step motor, and having a printing mechanism which employs a linear scanning thermal head to print letters and bar codes on paper sheets, labels or the like. In addition, the present invention includes a method for printing letters and bar codes by the above-mentioned thermal printer.
2. Description of the Prior Art
Commonly used thermal printers are generally ,equipped with a paper feeding mechanism which employs a step motor as the driving source. The paper feeding mechanism drives a heat sensitive sheet or a normal sheet and ink sheet (or ink ribbon in the case of a heat transfer type), through a platen, in a stepping motion. The platen then contacts with a linear scanning thermal head having heating elements are selectively energized in accordance with print data such as letters, bar codes or the like, which will then print each line.
In this type of thermal printer, it may sometimes be required to temporarily stop the paper feeding motion before the completion of printing and then restart the printing operation.
For example, a bar code printer, a kind of thermal printer, as shown in U.S. Pat. No. 4,795,281, is designed in such manner as that shown in FIG. 9 and FIG. 10(A) of the present specification, the thermal head 3 prints a bar code BC on a paper sheet 2 or a paper label 2a fed by the platen 1 and then, the printed bar code BC is read out by the scanner 4 which scans in a right angle direction with respect to the paper feeding direction, so as to verify the printed data with the input data. In this bar code printer, when the height H (as shown in FIG. 10) of the bar is too low to be scanned by the scanner 4 by the feeding of the paper sheet, the bar code printing operation is temporarily stopped to scan the preceding bar code because the printing position P and the scanning position S are isolated and then, the succeeding bar code printing operation is restarted.
Alternatively, when the verified data determines that the printed bar code is not completely formed (NG) even though the height H of the bar code is high enough to be scanned by the scanner 4 with the feeding of the paper sheet, the printing operation is immediately stopped. The scanner 4 scans the bar code again under the condition that the printing operation is stopped. When the verified data determines that the printed bar code is completely formed (OK), the printing operation is restarted to recontinue the printing of the bar code.
Furthermore, the thermal printer, as shown in FIG. 11, is designed in such manner that a paper sheet 2 passed through a printing position P is cut by a stationary cutter 5. In this printer, it is preferable that the paper feeding operation is temporarily stopped for the cutting operation. However, the printing operation is simultaneously carried out with the cutting operation because the printing position P is relatively isolated from the cutting position C. Thus, the printing operation is temporarily stopped as the paper feeding operation stops.
In these kind of printers whereby paper feeding operation should be stopped when any operation such as, the cutting operation, is carried out in the downstream side of the printing position, in accordance with the printing size or printing interval of the paper feeding direction, the printing operation may coincide with the time when the paper feeding operation is stopped.
Furthermore, in the upperstream side of the printing position P, it may sometimes be required to temporarily stop the paper feeding operation for the printing operation.
For example, as shown in FIG. 12, the heat transfer type thermal printer using an ink ribbon 6 includes a ribbon sensor 7 and a paper sensor 8 which respectively detect when the ribbon and paper run out. In this type of printer, the printing operation should be quickly stopped when the distance between the printing position P and the sensor 7 and 8 is short, the printing speed is fast, and the ribbon sensor 7 and/or the paper sensor 8 signal the end of either. If the printing operation is not stopped at once the same position at the ribbon will be repeatedly heated or the platen 1 is subjected to printing This condition may result in many problems.
In order to avoid such problems, the printing operation is repeated again from the beginning of the same page after changing the ribbon or the paper. However, this method tends to consume excess ribbon and paper, in order to prevent this from happening, the following control may be used.
Step numbers for feeding the paper from the printing position P to the ribbon sensor 7 or the paper sensor 8 are previously stored in a memory. When the ribbon sensor 7 and/or the paper sensor 8 output their signals, a control unit judges whether the printing operation can be performed to the end of the page or not, in accordance with the data stored in the memory and format length of the paper set in the printer. If possible, the printing operation will be restarted from the stopped position of the same paper.
Furthermore, as, shown in FIG. 12, a stamp 9 is arranged at the upstream side of printing position P to provide a mark prior to the printing. This stamping operation also needs to temporarily stop the paper feeding operation. This may interrupt the printing operation in accordance with the printing interval and printing size in the paper feeding direction.
In this kind of linear printing thermal printer, when the printing operation is restarted after temporarily,. stopping the paper feeding operation, the printing cannot be started exactly from the interrupted section of the previous print due to the relationship between the paper motion and the print-timing, and/or the radiation of heat from the thermal head. This may generate a linear print-gap in the lateral direction of the bar code, as shown in FIG. 10(B), after the interruption of the printing operation of the bar code, as shown in FIG. 10(A). Thus, the bar code with the linear print-gap may cause some problems.